1. Field of the Invention
The present invention relates to an improved method for producing a shaped, hollow resin article. More particularly, the present invention is concerned with an injection molding method for producing a shaped, hollow resin article by injecting a resin in a molten form into a mold cavity to form a molten resin mass, and introducing a pressurized fluid into the molten resin mass, wherein, during the introduction of the pressurized fluid into the molten resin mass, the temperature of at least one preselected portion of a back surface of the molten resin mass is maintained at a level which is higher by a specific temperature difference than the temperature of a front surface of the molten resin mass, causing the fluid to flow into the molten resin mass along a line substantially corresponding to a line along which the preselected portion of the back surface extends, so that the shaped, hollow resin article produced has a hollow in a position corresponding to the preselected portion of the back surface. By the method of the present invention, the flow of the pressurized fluid introduced can be surely led to an intended portion in a molten resin mass, so that the location of a hollow in the molten resin mass can be surely controlled and, therefore, a shaped, hollow resin article having a hollow of an orderly morphology can be produced. The present invention is also concerned with a mold for use in the method.
2. Discussion of Related Art
Conventionally, for the purpose of not only preventing the occurrence of sink marks of a resin on a local thick-walled section but also producing a light weight shaped resin article, there have been employed various injection molding methods (so-called gas injection molding methods) for producing a shaped, hollow resin article by injecting a resin in a molten form into a mold cavity to form a molten resin mass, and introducing a pressurized fluid into the molten resin mass (the so-called "gas injection molding methods" include not only a method in which a gas is used as the pressurized fluid but also a method in which a liquid is used as the pressurized fluid). In the field of gas injection molding, for leading the flow of the pressurized fluid to an intended portion in a molten resin mass so as to control the location of a hollow to be formed in the molten resin mass, the following methods and molds suitable therefor have conventionally been proposed.
(1) A method in which a molten resin mass having continuous thick-walled sections of rib structures connected to a gate for introducing a pressurized gas is formed, and a pressurized gas is introduced into the thick-walled sections, and a mold suitable therefor (see Unexamined Japanese Patent Application Laid-Open Specification No. 63-268611, which corresponds to U.S. Pat. No. 4,923,666).
(2) A method in which use is made of a mold provided, at a portion thereof corresponding to a thick-walled section of a shaped resin article to be produced, with a heat insulating member or heater which is capable of lowering the cooling rate of the molten resin at the thick-walled section as compared to the cooling rate of the molten resin at portions other than the thick-walled section (see Unexamined Japanese Patent Application Laid-Open Specification No. 4-62125).
(3) A method in which use is made of a mold, the overall inner wall surface (defining a mold cavity) of which is coated with a heat resisting, heat insulating resin film in a thickness of 0.001 to 2 mm (see Unexamined Japanese Patent Application Laid-Open Specification No. 5-245881). The specification describes that this method renders it possible to solve problems, such as poor reproduction of the profile of the mold inner surface on a resultant molded product and occurrence of a hesitation mark, which are likely to be caused in forming a hollow in a thick-walled section of the resultant molded product.
(4) A method in which a molten resin mass having a continuous thick-walled section of rib structure connected to a gate for introducing a pressurized gas is formed, and a pressurized gas is introduced into the thick-walled section, and in which use is made of a mold having a heat insulating member at a portion of the mold which corresponds to the above-mentioned thick-walled section, wherein the overall inner wall surface of the mold is coated with a heat insulating film (see Unexamined Japanese Utility Model Application Laid-Open Specification No. 6-34927).
(5) A method in which use is made of a nozzle (for injecting the molten resin mass) having a heater embedded in an inner wall of the nozzle longitudinally thereof so as to cause a temperature difference in the inner wall of the nozzle, and a molten resin is injected into a mold cavity through the nozzle to thereby cause a temperature difference in the injected molten resin mass, and a pressurized gas is introduced into the molten resin mass at a high temperature region thereof. This method utilizes the fact that the viscosity of the high temperature region of the molten resin mass is less than the viscosity of the low temperature region of the molten resin mass, so that the resistance to the flow of the pressurized gas in the high temperature region of the molten resin mass is lower than in the low temperature region. Also disclosed are a method and a mold suitable therefor in which a mold inner surface defining a mold cavity is radially divided into a plurality of sections taking a sprue as a center of the radiation, and heating sections and cooling sections are alternately disposed to cause a temperature difference between adjacent sections (corresponding to the adjacent sections of the mold inner surface) of an injected molten resin mass, so that a pressurized gas can be led to high temperature sections of the molten resin mass in which the viscosity of the molten resin mass is low and therefore the flow resistance to the pressurized gas is low. (see Examined Japanese Patent Application Publication Specification No. 61-53208).
The basic concept of each of the above-mentioned conventional methods resides in that the temperature of a preselected local portion of the molten resin mass in which portion a hollow is to be formed is maintained at a level which is higher than the temperature of a portion surrounding the local portion, to thereby render the preselected local portion relatively low in flow resistance, so that a pressurized fluid can be led into the preselected local portion having a relatively low flow resistance. In order to achieve such a temperature difference in a molten resin mass, a thick-walled section is formed in the molten resin mass in methods (1), (2), (3) and (4) above and, in method (5), differing from those methods, a temperature difference in the molten resin mass is generated by specifically designing the inner wall of the nozzle for injecting a molten resin or the mold inner surface defining the mold cavity.
However, the above conventional methods have the following disadvantages.
In the methods (1), (2), (3) and (4) above, it is necessary to form a thick-walled section in the molten resin mass for leading a pressurized gas thereinto, which necessarily restricts the design of a shaped resin article. Further, in the methods (1) and (2), due to the difference in thickness of the resin by forming a portion (i.e., thick-walled section) at which a volume contraction is large, the surface appearance of the resultant shaped resin article at a portion corresponding to the thick-walled section becomes poor, so that finishing of the article, such as coating, becomes necessary. Furthermore, in the methods (3) and (4), in order to improve the surface appearance of the resultant shaped resin article, the overall inner wall surface of the mold has a thin coating of a heat insulating material. However, such a mold is expensive and the durability of the thin coating is unsatisfactory for the mass production of shaped resin articles.
In method (5) above, depending on the shape of an intended shaped article, in some cases, it is likely that a pressurized gas cannot be surely led to an intended portion in a molten resin mass and, even if the gas is surely led to an intended portion, the surface of a local outer portion of the resultant shaped article which corresponds to the hollow portion in the shaped article is different in gloss and luster from the surface of a portion surrounding the local outer portion, so that the surface appearance of the shaped resin article produced becomes poor.
As described in detail below, the present inventors have conducted extensive and intensive studies to elucidate the reason why a non-uniformity (which is causative of a deterioration of the surface condition) occurs in the gloss and luster of a shaped, hollow resin article. As a result, the present inventors have found that the occurrence of the non-uniformity in gloss and luster is largely influenced by the thickness of the shaped, hollow resin article at its portion having a hollow therein and by a temperature difference between a front surface and a back surface of a molten resin mass at its portion in which a hollow is to be formed. Specifically, the present inventors have found that the mechanism by which a non-uniformity in the gloss and luster of a shaped, hollow resin article occurs is varied depending on the thickness of the shaped, hollow resin article at its portion having a hollow therein. As a result of further studies, the present inventors have also found that occurrence of a non-uniformity in the gloss and luster of a front surface of a shaped, hollow resin article can be prevented by regulating the temperature difference between the front and back surfaces of a molten resin mass at its portion in which a hollow is to be formed, i.e., by regulating the cooling conditions of the front and back surfaces of the molten resin mass, in accordance with the thickness of the shaped, hollow resin article at its portion having a hollow therein.
However, as described above, the above-mentioned prior art techniques only attempt to maintain a portion of the molten resin mass at which a hollow is to be formed at a temperature higher than that of other portions of the molten resin mass, and the prior art techniques have no teaching concerning the above-mentioned regulation of the temperatures of the front and back surfaces of a molten resin mass according to the thickness of the molten resin mass at its portion in which a hollow is to be formed. Further, there is no description in any of the above-mentioned Japanese patent documents (1) through (5) about producing a temperature difference between the front and back surfaces of a molten resin mass at its portion in which a hollow is to be formed.